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EXOFOT - EXOSAT Final Observation Tapes


The EXOFOT database contains the log of the EXOSAT final observation tapes (FOTs). This database helps to locate the desired EXOSAT data file within the HEASARC anonymous FTP account. The database itself contains a minimum number of parameters regarding the observation which correspond to the first 255 bytes which label the FOT. Those parameters are target name, experiment, and start and stop time. The other parameters in the database are more related to the organization of the archive.

Getting the Data

The data can be accessed through the anonymous FTP account on `legacy.gsfc.nasa.gov`. Because of the large volume of the mission, data have been compressed using the public software `zoo`. Before retrieving the data, the user should remember that the compression factor can be up to six times the current size of the file. If the user does not have the `zoo` software, a version for Unix machines can be retrieved under this ftp account in `software/zoo`.

The data format is the original binary format, the description of which can be found in the FOT-handbook. This document was distributed by ESA, with the data to the original PI. The document does not exist in electronic form; however, a printed copy may be obtained from the HEASARC upon request.

Historical View

The European X-ray Observatory Satellite (EXOSAT) was operational from May 1983 to April 1986 and in that time made 1780 detailed observations of a wide variety of astronomical objects. The scientific payload consists of two energy telescopes (LEIT), a medium energy proportional counter (ME), and a gas scintillator proportional counter (GSPC).

The data acquired for the four instruments were distributed to observers as FOT (Final Observation Tape) in a form of 1600 BPI tape. A single FOT contains data for only one instrument, and a FOT (observation) could consist of more than one 1600 BPI tape, which generally contained data from the pointed observation plus part of the in-going and out-going slews (only for the GSPC and ME instruments).

Since 1992, the total amount of ~8643 tapes (1600 BPI), for a volume of 150 Gbytes of data, has been archived at ESRIN/ESA, on an optical disk, by the ESIS project. An exact copy has been given to HEASARC/GSFC (Greenbelt, MD, USA). From ESRIN the data arrived at HEASARC on magnetic media, each containing a number of FOTs. At HEASARC the magnetic tapes were read into a jukebox organized into directories named from the magnetic tape number delivered from ESRIN.

ESIS could not recover and archive the complete mission because some of the earliest 1600 BPI tapes (now over ten years old) were badly damaged. Currently there are about 50 observations for which data recovery was not possible. Letters have been sent to the original PIs of those observation in the hope that their copies of the data were better maintained. For a number of observations (in particular for the early ones), more than one FOT is available. This is because the FOT was regenerated for various reasons. The record seems to be missing as to which is the correct FOT.

Searching for Data

The following three parameters are used to identify the data set within this database: target name, experiment, and start and stop time. The user should be aware that a) within this database is not possible search by coordinates, and b) one entry does not necessary correspond to a single entry within the EXOSAT observation log. To search for your data within this database, it is recommended to use first the EXOSAT observation log database (EXOLOG) to retrieve the experiment, target name, start and stop time. With this information, search within this database using the parameters TIME and/or TIME END using a time window which includes the value found in the EXOLOG database for the parameters FOT TIME and FOT END. In this way at least three (four early in the mission when the LE2 was still operating) entries should be selected, one for each instrument (LE1, LE2, ME, GSPC).


The FAI number.

The FAI number is a parameter which connects the HEASARC database with the ESRIN database. It is useful to HEASARC personnel to exchange information about particular datasets with ESRIN.

Each data file (FOT) that can be retrieved from this database contains data from only one instrument. The values for the `instrument` parameter are `LE1`, `LE2`, `ME` or `GSPC`. The two LE telescopes are treated as separate instruments because the filters and other instrument modes can be different.

The `name` parameter gives the target name. This name usually corresponds to the target name as specified by the original observer. If a source has been observed by different observers, it may be entered under two or more names. There are a number of `special` strings for the target field of which the user must be aware. Those are as follows: a) 2 TARGETS, b) 3 TARGETS, c) 4 TARGETS, d) ???? and e), a blank string.

In the a), b), and c) cases, the file associated with the entry in the database contains observations for more than one celestial object. In the d) and/or e) cases, either it was not possible to retrieve the value from the tape header or that value was found blank.

The `Time` parameter refers to the start time within a FOT. The data in the FOT can be related to one or more entries in the EXOLOG database. For those cases in which the `Time` parameter does not match precisely the start time of an entry in the EXOLOG database, it is recommended to retrieve data such that start time for the identify entry in the EXOLOG is included in the `Time` parameter.

The time is listed as `yr.day`, where `yr` is the last two digits of the year and `day` is the day of the year; for example, day 300 of 1984 is 84.300.

End Time of Observation

Date of ESOC Transcription

The string in the `FOTID` field is the identifier assigned to the FOT by ESOC/ESA (Germany).

This parameter flags data sets in which errors occurred during the reading transcription procedure of the 1600 BPI tape into the optical juke-box. If the parameters is set `YES` it is always followed by the number of bad blocks which occur in the dataset. The parameter is set to `NONE` if the data set is not corrupted.

This parameter gives the directory name where the file is located under the anonymous FTP account on the `legacy` machine. The parameter value `tape#` ranges from 1 to 49. Note that this is not the full path. In the anonymous FTP account, the directories (this parameter) can be found under `exosat/data/raw`. For example, if the file is located in directory `tape5`, under the anonymous FTP account the full path to the data will be `exosat/data/raw/tape5`.

Name of Archive File

This is file size in bytes of the compressed file. The user would be aware that the compression factor can be as great as six times the uncompressed value. The uncompressed file size is written in the parameter `File Bytes`. Within a `zoo` file, multiple files can be present if the original FOT observation consisted of more than one 1600 BPI tape.

Compression Factor

An observation (FOT) distributed on magnetic tape could consist of more than one magnetic 1600 BPI tape. If multiple tapes were required to store data for one observation (FOT), each of them has been archived as a separate file. In the compression procedures, all of the files that belong to an observation have been `zoo'd` together. The parameters `NFiles` gives the number of files compress within a `zoo` file.

`File Names` gives the listing of names of each single file that can be found in the `zoo` file. The number of names should match the parameter `NFiles`.

Gives the original size in bytes of the uncompress data. More than one value is stored if multiple files have been `zoo'd` together.

Contact Person

Questions regarding the EXOFOT database table can be addressed to the HEASARC User Hotline.
If you have any problems, please consult the help page or mail ledas-help@star.le.ac.uk